The present invention relates generally to intracavity lasers incorporating a birefringent nonlinear element and more particularly to an alignment technique for such a laser which decouples one alignment adjustment from one or more other alignment adjustments in a way which significantly reduces the complexity of the overall alignment procedure, thereby reducing manufacturing costs.
The construction of any laser must include a step in which the laser is aligned in such a way that one or more selected wavelengths of light experience gain during retrace between the mirrors which serve to define the resonant cavity of the laser. When other elements are also included in the cavity, as an example, a nonlinear birefiingent element used for purposes of providing second-harmonic generation, other conditions must also be satisfied. A birefringent nonlinear element, when used for second harmonic generation, is properly positioned in the light path when, in addition to considerations of the optimum polarization vector orientation of the fundamental lasing radiation with respect to the crystal axes, the light path taken by the fundamental(s) through the birefringent nonlinear element cooperates with the overall light path to provide for retracing of the fundamental(s), the phase retardance properties of the birefringent nonlinear element (acting in concert with any polarization sensitive elements) impart minimal/acceptable losses to the lasing fundamental(s) and the phase matching properties of the nonlinear element between the fundamental(s) and one or more desired harmonics are at a peak (i.e., conversion of light from the fundamental(s) to the desired harmonic wavelength(s) is maximized in order to provide an acceptable output power at the harmonic wavelength(s)). Many applications for lasers which emit light at a harmonic of a fundamental lasing frequency, e.g. frequency-doubled lasers, require that the output be highly stable across a large frequency bandwidth. Frequency-doubled lasers which exhibit periodic fluctuation of intensity of greater than 0.5% at any frequency up to 10 or 20 Mhz are commonly referred to as being `noisy` and unsuitable for many applications. These fluctuations are an undesirable side effect of the coupling of various fundamental lasing modes operating within the cavity. The only prior art solutions to this problem are to force the laser to operate in only a single mode, or to operate in such a large number of modes that the fluctuations cancel one another out on a statistical basis.
In a majority of prior art lasers utilizing a birefringent nonlinear element, the latter element was arranged in the light path such that its parallel input/output surfaces were normal to the light path. This arrangement was thought to be advantageous since lasers such as frequency doubled intra-cavity lasers are extremely intolerant of losses due, for example, to reflections. Angling the nonlinear element with respect to the light path would result in reflecting at least a portion of the fundamental wavelength out of the light path, constituting an unacceptable loss at the fundamental wavelength which, of course, translates into a loss at the harmonic wavelength. At the same time, however, it should be appreciated that certain problems were introduced by placing the birefringent element in normal incidence to the beam. For example, the retardance properties of the material could not be controlled accurately by precise manufacturing and could only be affected by temperature. The amount of control available through temperature adjustment was dependent upon the length of the birefringent element. Moreover, the parallel surfaces of the element, in an orientation which is normal to the light path, may cause intra-cavity etalon or sub-cavity effects resulting in undesirable noise imposed upon the output wavelength(s).
More recently, anti-reflective coatings have been provided on the light input/output surfaces of the nonlinear element which lower reflections from the input/output surfaces to an acceptable level even in an angular orientation with respect to the light path. Thus, lasers can now be built with components placed at an angle to the beam path to avoid the problems caused by normal interfaces. In addition, an extra means of adjustment, tilt angle, is now available to optimize alignment. Unfortunately, achieving a satisfactory alignment using the tilt angle and temperature adjustments can be an extraordinarily difficult task since the laser behaves in a seemingly unpredictable manner as the adjustments are performed. That is, for example, as the tilt angle or temperature is adjusted very slightly with the laser operating at a single wavelength and output intensity, the output may jump wildly to another wavelength or combination of other wavelengths with an output intensity that is much different than the pre-adjustment intensity. In instances where the post-adjustment output power is greater than the pre-adjustment power, one must again check for low noise and good spatial mode quality in the laser's output; instances in which the new output is lower in power are not of great commercial interest. This unpredictable behavior contributes significantly to manufacturing costs by imposing an alignment procedure which typically requires significant skill and a substantial, but unavoidable period of time to perform for reasons which will be made evident at an appropriate point hereinafter.
The present invention provides a highly predictable alignment technique which utilizes the tilt adjustment in combination with other adjustments in a heretofore unknown and highly advantageous way. Due to the predictable behavior of the laser during the use of this technique, manufacturing costs are thereby directly reduced.